This is done in master:
http://git.fedorahosted.org/git/?p=rhq/rhq.git;a=commitdiff;h=6eb0643
The new trait is called 'multicastAddress' and is defined on the Standalone Server and Managed Server types. A value will only be returned for the trait for a server that is in HA/cluster mode. The value format is "HOST:PORT" (e.g. "230.0.0.4:45688").